The subject matter disclosed herein relates to turbine casings and nozzle diaphragms for industrial gas and wind turbines, and more particularly, to cast iron near-net shape turbine casing and nozzle diaphragm preforms.
Turbine casings for operating at elevated temperatures have generally been restricted to alloy steel castings or fabrications. Traditional ferritic ductile irons are less costly than alloy steels, but typically have had an inadequate combination of properties, thereby precluding their use in advanced gas turbine compressor discharge and turbine shell casings and other components, such as nozzle diaphragms. One of the limiting aspects has been related to the fact that these components have been made as sand castings. Finish machined conventional sand castings used for turbine components, such as casings and nozzle diaphragms, must have complex features added to them by machining. Due to the nature of conventional sand casting processes, which are currently used to cast the cast iron casings and nozzle diaphragms used in gas turbines and wind turbines, these features, including vane slots, bolt holes and various seals, depend on extensive machining upon completion of the casting process. It is not uncommon to find that the machining processes costs considerably more than the casting. However, from a metallurgical perspective, increasing the casting size to provide machining allowances significantly decreases the structural integrity of the cast parts. Larger castings with more machining allowances take longer to solidify and cool, which can cause degenerate graphite formation in ductile iron castings. Further, the larger castings typically require more risers or reservoirs for the molten metal to increase castability. However, the addition of more risers also tends to increase the likelihood of producing degenerate forms of graphite, which are known to reduce the elongation and fatigue properties resulting in reduced operating lifetimes. Also, desirable fine grain structures are typically found adjacent to as-cast surfaces. However, current sand cast iron components used in gas and wind turbines are heavily machined removing the desirable fine grain structures and frequently exposing undesirable internal microstructural features and volumetric defects, such as internal microporosity and degenerate graphite forms. Therefore, it is desirable to provide cast iron turbine components that significantly reduce or eliminate machining operations, provide desirable fine grain microstructures and avoid the creation and exposure of undesirable internal microstructural features.